Method for supporting sensitive workpieces during processing

ABSTRACT

A method of supporting an object having an even contact surface during manufacturing operations, by urging it to a rigid substrate, the contact surface facing a supporting surface of said substrate, for use in an environment containing particles that are likely to be entrapped between the object and the substrate. The method includes provision of a supporting surface in the form of a plurality of bulges disposed on the substrate with depressions therebetween. The bulges have crests with rounded profile enabling the entrapped particles to fall into the depressions, thereby reducing the probability of causing mechanical damage to the object and/or of a faulty manufacturing operation on the object.

FIELD OFF THE INVENTION

[0001] This invention relates to supports for holding a workpiece duringits fabrication and, more particularly, to supports for workpieces withsensitive surface, such as panels processed in printing or imaging.

BACKGROUND OF THE INVENTION

[0002] Through the fabrication processes of a double-sided panel orother workpiece like a printing plate, chip or wafer, printed circuitboard (PCB) or printed wiring board (PWB) etc., there are operationsthat are performed separately on each of the two surfaces of the panel.In order to enable automated and accurate processing operation to beperformed on such panel, it must be positively positioned and securelyfixed to a relatively rigid substrate. When positioned in such a manner,the panel faces the substrate with its one, contact surface, whileanother, working surface is subjected to the processing operation. Whenthe panel is a relatively thin object which does not retain itsplanarity while being disposed on the substrate, it must be smoothed,for example by a rubber-coated roller pressing the panel, to fit theshape of the substrate. Then, a negative air pressure is created betweenthe panel and the substrate in order to fix the panel securely onto thesubstrate. The forces between the contact surfaces, resulting from thesmoothing operation and the negative pressure, are controlled to besmaller than the scratch resistance of the contact surface of the panel.However, only average forces are controlled, accounting for the appliedworking forces, pressures and total contact surface area, while localforces acting on local contact areas are uncontrollable.

[0003] During successive fabrication steps of the panel, there alwaysoccurs disintegration of the construction material, especially when thepanel is made of a composite material such as laminated glass epoxy, andsmall particles of this material cling to the surfaces of the substrateand/or of the panel. This is an inherent phenomenon of the fabricationprocess that can hardly be avoided. Also, particles created during theprocessing of other panels as well as during various machine movements,remain in the manufacture environment and some of them get attached tothe panel Even in a controlled environment there are dust andcontamination particles brought in by the personnel and materials thatfall on the substrate attracted by its static electrical charge.

[0004] A particle may sometimes originate from a glass fiber, steel chipor the like and, therefore, may be very hard. FIG. 1 illustrates such aparticle 10 when trapped. between the surface of the substrate 12 andthe contact surface 13 of the panel 14 that is supported by thesubstrate. Due to its small size and irregular shape, the particle 10has a very small contact area 16 with the panel contact surface. Such aparticle also lifts the panel locally off the surface of the substrateso that the reaction to the urging pressure P_(urge) is distributed overthe small area 16 and, therefore, high local pressure is developed,which can surpass the scratch resistance and damage the panel. Due tothe local raising-up of the panel, a manufacture operation on theworking surface 18 may fail.

SUMMARY OF THE INVENTION

[0005] In accordance with the present invention, there is provided amethod of supporting a workpiece with a flat and scratch-sensitivecontact surface during manufacturing operations, by urging it to a rigidsubstrate, where the contact surface of the workpiece faces a supportingsurface of the substrate. The method includes provision of a supportingsurface on the substrate, comprising a plurality of bulges withdepressions therebetween. The bulges have crests with rounded profilethat enables a particle entrapped between the workpiece and thesubstrate to fall into a depression, and the depth and the width of thedepressions are such as to accommodate the particle at least partially.This structure reduces the probability of causing mechanical damage tothe workpiece and/or of a faulty manufacturing operation on the samethat may cause resection of the workpiece.

[0006] The bulges preferably are in the form of elongated ridges thatmay be arranged in various patterns on the substrate. The patterns maybe formed by processing the substrate surface itself or by attaching alayer of material with preformed ridges.

[0007] The method of the present invention is especially advantageous inthe production of printed circuit boards and printed wiring boards, whenprocessed by imaging or printing operations. It may be used for otherworkpieces of any shape having at least one flat sensitive surface.

BRIEF DESCRIPTION OF TIME DRAWINGS

[0008] In order to understand the invention and to see how it may becarried out in practice, a number of embodiments will now be described,by way of non-limiting example only. with reference to the accompanyingdrawings, in which:

[0009]FIG. 1 is a schematic sectional view showing a hard particletrapped under a flat panel on a conventional substrate;

[0010]FIG. 2A is a schematic sectional view of a flat panel held on asubstrate according to the present invention;

[0011]FIG. 2B shows an enlarged detail B of FIG. 2A;

[0012]FIG. 3 shows a regular orthogonal network pattern of ridges on asubstrate according to one embodiment of the present invention;

[0013]FIG. 4 shows a helical pattern of ridges wound on a drum substrateaccording to another embodiment of the present invention;

[0014]FIG. 5 shows a wave-like pattern of ridges on a substrateaccording to a further embodiment of the present invention; and.

[0015]FIG. 6 shows a random pattern of straight ridges on a substrateaccording to a still further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] According to the present invention, as illustrated by FIG. 2A, aprotective supporting surface 20 is provided on a substrate 28 for thesupport of a panel (workpiece) 14 having a contact surface 13. Theprotective surface 20 is uneven, having the form of a plurality ofbulges 22 and depressions 24 remaining therebetween. Preferably, thebulges are in the form of elongated ridges, with crests 26 havingrounded profiles.

[0017] As seen in FIGS. 2A and 2B, the total contact area between thecontact surface 13 of the panel 14 and the crests 26 of the bulges 24 isconsiderably smaller than the nominal area of the panel contact surface13 projected onto the substrate. 28. Moreover, as shown in FIG. 2B, thelocal pressure is not quite evenly distributed across each zone ofcontact. Thus, the local pressures P_(local) on the contact surface 13of the panel are a few times larger than the uniformly distributedurging pressure P_(urge) However, the appropriate selection of theprofile form and of tie width of the depressions allows average localpressures P_(local) to be controlled and kept well below the scratchresistance of the panel. In particular, in operation, with thesupporting surface 20 designed according to the present invention, aparticle that may be trapped between the panel 14 and the substrate 28,as shown in FIG. 2A, may behave in one of the following ways:

[0018] 1. A particle 30 may originally appear to be entirelyaccommodated in the depression 24 and, being smaller than tiledepression, not touch the panel at all.

[0019] 2. A particle 32 may originally appear to be not entirelyaccommodated in the depression having a width or height greater thanthat of the depression. In this case, the particle 32 will, contact thepanel but the local pressure will be smaller than the pressure thatwould have been created if such a particle were trapped between aconventional substrate and the panel, as shown in FIG. 1.

[0020] 3. A particle such as particle 30 or 32, may originally bedisposed totally on top of the bulge 22 but will most probably be pushedoff the side of the bulge into a depression of the substrate and willbehave in either of die above ways.

[0021] 4. In the least probable case, if a particle 34 remains on thebulge 22, the local pressure will be about the same as die pressure thatwould have been created by such a particle trapped between smoothsurfaces of the substrate and the panel.

[0022] The statistical probability of a particle falling into each ofthe above cases is affected by the height, crest profile shape andspacing of the bulges and, therefore, may be controlled through thedesign and material selection of the protective surface.

[0023] The protective supporting surface is preferably designed as acombination of ridges with uniform height disposed on a basic surface ofthe substrate. The protective surface may also be fabricated on aseparate interface sheet and then attached to the substrate.

[0024] The following criteria are to be considered in the design of aprotective surface according to the present invention:

[0025] the ratio of the contact area of the ridges to the nominal areaof the contact surface, which should ensure that local pressures are atleast an order of magnitude lesser than the scratch resistance of thesensitive contact surface of the workpiece.

[0026] the ridge width w and the shape of their crests and their radiishould be such as, on the one hand, to yield more uniform distributionof local pressures in the contact area between the supported workpieceand the ridge (see FIG. 2B), and on the other hand, to increase theprobability for an entrapped particle to slip off a ridge into adepression;

[0027] the ridge spacing d and ridge height h should be such as toaccommodate most of particles, but they should meet the condition toavoid deflection of the workpiece that would cause, e.g. in the case ofprinting operation, distortion of the pattern imaged on the panel.

[0028] In practice, it is preferable to have the ridge spacing d largerthan the ridge width w, in order to accommodate most of tiecontaminating particles.

[0029] In the manufacture of printing plates, chips, wafers, PCB or PWB,typical particles have dimensions between 3 to 50 μm. A protectivesupporting surface suitable for such manufacture should preferably haveridges with crest radius of curvature at least 0.1 mm, especially at thepoints of maximum height. The depression profile is preferably about 0.1mm deep.

[0030] The experiments show that the use of protective supportingsurfaces according to the present invention, allow to reduce rejectionof PCB-s due to their backside scratches by a factor of 3.

[0031] Examples of protective supporting surfaces in accordance with thepresent invention are presented below with reference to FIGS. 3 to 6.

[0032]FIG. 3 illustrates a layer of protective media in the form of a.Teflon sheet having a smooth surface on one side and a network ofprotruding orthogonal ridges on the other side. The ridges may be glassfibers that are overcoated with Teflon. The Teflon sheet may be of thetype 216AP with adhesive back TY, made by Tygaflor company in the UK.The sheet is attached with its adhesive surface to the surface of thesubstrate. The sheet is flexible and may be attached either to a flatbedsubstrate or to a rotary drum substrate. The sheet also may be cut intoseveral pieces for the use as a protective surface of the presentinvention to support each processed panel, whereby more than one panelmay be processed on the same substrate. Such sheets are easilyreplaceable by peeling them off the substrate when they become worn ordirty. Due to the Teflon overcoat, the ridges on the sheets are softerthan the substrate and their flexibility contributes to a betterdistribution of the pressure, which prevents or reduces the damage tothe panel whenever confronted by a particle.

[0033] The pattern of FIG. 3 may be also obtained by spreading a Nylonnet or a domestic fly net having 0.5×0.5 mm up to 1.5×1.5 mm openings onthe substrate.

[0034]FIG. 4 illustrates a protective supporting surface on a rotarydrum substrate. The surface is obtained by helically winding a 0.2-0.4mm. diameter Nylon fishing line around the drum, the line forming theridges.

[0035]FIG. 5 illustrates a pattern of wave-like protrusions embossed ona stainless steel sheet, which may be mounted by mechanical clamps ontothe substrate.

[0036] The pattern of the ridges or grooves on the substrate surface mayhave also irregular or random character, as shown in FIG. 6.

[0037] A protective supporting surface in accordance with the method ofthe present invention may be created by processing the surface of thesubstrate rather than using a separable media. Ridges on this surfacemay be obtained by cutting grooves thereon or by electrochemicalmachining that relieves material in a network shape or by vibrationrolling. In this manner, a textured protective surface may be formed bya network of continuous grooves with protruding isles therebetween, asopposed to the network of ridges.

[0038] Still another kind of protective supporting surface may beobtained by applying to the substrate surface a non-smooth paint, forexample of orange-peel texture, or by coating the surface withnon-smooth texture coating.

1. A method of supporting an object having an even contact surface byurging it to a rigid substrate during a manufacturing operation in anenvironment containing particles that are likely to be entrapped betweensaid object and said substrate, said method including provision on saidsubstrate of a supporting surface in the form of a plurality of bulgesdisposed on said substrate with depressions therebetween, said bulgeshaving crests with rounded profile enabling at least a part of saidentrapped particles to fall into said depressions, thereby reducing theprobability of causing mechanical damage to said object and/or of afaulty manufacturing operation on said object.
 2. A method according toclaim 1, wherein the depth and the width of said depressions are such asto at least partially accommodate most of said particles.
 3. A methodaccording to claim 1, wherein said supporting surface of said substrateis formed as a separate interfacing element attached to said substrate.4. A method according to claim 3, wherein said interfacing elementincludes a layer of adhesive material.
 5. A method according to claim 3,wherein said separate interfacing element is made of material which issofter than said substrate.
 6. A method according to claim 2, whereinsaid object is a layer of a printed circuit board (PCB) or of a printedwiring board (PWB).
 7. A method according to claim 2, wherein saidsubstrate is a rotary drum of an imaging or a printing machine and saidobject is flexible.
 8. A method according to claim 2, wherein saidsubstrate is a flat bed of an imaging or a printing machine.
 9. A methodaccording to claim 1, wherein said bulges are in the form of elongatedridges.
 10. A method according to claim 9, wherein said elongated ridgesform a regular network on the substrate surface.
 11. A method accordingto claim 10, wherein said network is obtained by using a mesh attachedto the substrate surface.
 12. A method according to claim 9, whereinsaid elongated ridges form a pattern of continuous straight or curvedlines running without intersections on the substrate surface.
 13. Amethod according to claim 12, wherein said pattern is formed by laying acontinuous fiber or wire on the surface of said substrate.
 14. A methodaccording to claim 13, wherein the substrate is a body of rotation andsaid pattern is formed by winding a thread or wire along a helical lineover said body.
 15. A method according to claim 1, wherein the roundedprofiles of said crests have at least 0.1 min radius.
 16. A methodaccording to claim 1, wherein said depressions are in the form ofelongated grooves.
 17. A method according to claim 16, wherein saidelongated grooves form a network on the substrate surface.
 18. A methodaccording to claim 17, wherein said network form a texture on thesubstrate surface, similar to orange-peel texture.
 19. A methodaccording to claim 1, wherein said bulges and depressions are created byprocessing said supporting surface of said substrate.